Plasma has long been employed for processing substrates (e.g., wafers, flat panel displays, liquid crystal displays, etc.) into electronic devices (e.g., integrated circuit dies) for incorporation into a variety of electronic products (e.g., smart phones, computers, etc.).
In plasma processing, a plasma processing system having one or more plasma processing chambers may be employed to process one or more substrates. Plasma generation may employ capacitively coupled plasma, inductively coupled plasma, electron-cyclotron technology, microwave technology, etc.
In an example capacitively coupled plasma processing chamber, the wafer may be disposed on a work piece holder, also known as a chuck. Generally speaking, the chuck functions as a lower electrode, which may be supplied with one or more radio frequency (RF) signals. Another electrode, known as the upper electrode, may be disposed in a spaced-apart relationship above the substrate. The gap between the substrate upper surface and the lower surface of the upper electrode typically forms a plasma generation region. In the typical scenario, the upper substrate is typically grounded, and the RF energy provided to the lower electrode is capacitively coupled to the plasma during substrate processing. That is, when reactant gases are released into the plasma generation region through gas feed tubes built into the upper electrode, the RF energy may couple with the released reactant gas to ignite and sustain a plasma in the plasma generation region for substrate processing.
In some plasma processing systems, the upper electrode may alternatively or additionally be energized with RF energy. The RF signal applied to the upper electrode may have the same or a different. RF frequency relative to the RF frequency of the RF signal supplied to the lower electrode.
One challenge confronting designers of plasma processing systems is the need to confine the plasma to the plasma generation region at all times and especially during plasma processing. In plasma processing chambers where the upper electrode is energized with RF energy, the need to confine plasma is particularly acute.
This is because when the upper electrode is energized with RF energy, an electric field is set up between grounded chamber components and the RF-energized upper electrode. This electric field may cause unwanted ignition of plasma from reactant gases in the gas feed passages which, as previously mentioned, are employed to provide reactant gases to the plasma generation region.
To elaborate on this phenomenon, it should be noted that the chamber walls and many portions of a typical plasma processing chamber are typically grounded for safety reasons, among others. When the upper electrode is provided with RF power, an electric field exists between the RF-powered upper electrode and the grounded chamber components above it. Reactant gases flowing in gas passages or tubes that traverse this electric field may be sufficiently excited to form a plasma inside the gas passages or in interstitial volumes between upper electrode layers and parts. This plasma is unintended and undesirable since its presence introduces unpredictability to the process and thus is often detrimental to the etch result. Furthermore, the unwanted plasma often accelerates upper electrode component erosion, leading to premature upper electrode or plasma processing system failure.
Controlling this undesirable plasma formation is one of any goals of embodiments of the present invention.